src/doc/rust-by-example/src/trait/derive.md

   1 # Derive
   2
   3 The compiler is capable of providing basic implementations for some traits via
   4 the `#[derive]` [attribute][attribute]. These traits can still be
   5 manually implemented if a more complex behavior is required.
   6
   7 The following is a list of derivable traits:
   8 * Comparison traits:
   9   [`Eq`][eq], [`PartialEq`][partial-eq], [`Ord`][ord], [`PartialOrd`][partial-ord]
  10 * [`Clone`][clone], to create `T` from `&T` via a copy.
  11 * [`Copy`][copy], to give a type 'copy semantics' instead of 'move semantics'
  12 * [`Hash`][hash], to compute a hash from `&T`.
  13 * [`Default`][default], to create an empty instance of a data type.
  14 * [`Debug`][debug], to format a value using the `{:?}` formatter.
  15
  16 ```rust,example
  17 // `Centimeters`, a tuple struct that can be compared
  18 #[derive(PartialEq, PartialOrd)]
  19 struct Centimeters(f64);
  20
  21 // `Inches`, a tuple struct that can be printed
  22 #[derive(Debug)]
  23 struct Inches(i32);
  24
  25 impl Inches {
  26     fn to_centimeters(&self) -> Centimeters {
  27         let &Inches(inches) = self;
  28
  29         Centimeters(inches as f64 * 2.54)
  30     }
  31 }
  32
  33 // `Seconds`, a tuple struct no additional attributes
  34 struct Seconds(i32);
  35
  36 fn main() {
  37     let _one_second = Seconds(1);
  38
  39     // Error: `Seconds` can't be printed; it doesn't implement the `Debug` trait
  40     //println!("One second looks like: {:?}", _one_second);
  41     // TODO ^ Try uncommenting this line
  42
  43     // Error: `Seconds` can't be compared; it doesn't implement the `PartialEq` trait
  44     //let _this_is_true = (_one_second == _one_second);
  45     // TODO ^ Try uncommenting this line
  46
  47     let foot = Inches(12);
  48
  49     println!("One foot equals {:?}", foot);
  50
  51     let meter = Centimeters(100.0);
  52
  53     let cmp =
  54         if foot.to_centimeters() < meter {
  55             "smaller"
  56         } else {
  57             "bigger"
  58         };
  59
  60     println!("One foot is {} than one meter.", cmp);
  61 }
  62 ```
  63
  64 ### See also:
  65 [`derive`][derive]
  66
  67 [attribute]: attribute.html
  68 [eq]: https://doc.rust-lang.org/std/cmp/trait.Eq.html
  69 [partial-eq]: https://doc.rust-lang.org/std/cmp/trait.PartialEq.html
  70 [ord]: https://doc.rust-lang.org/std/cmp/trait.Ord.html
  71 [partial-ord]: https://doc.rust-lang.org/std/cmp/trait.PartialOrd.html
  72 [clone]: https://doc.rust-lang.org/std/clone/trait.Clone.html
  73 [copy]: https://doc.rust-lang.org/core/marker/trait.Copy.html
  74 [hash]: https://doc.rust-lang.org/std/hash/trait.Hash.html
  75 [default]: https://doc.rust-lang.org/std/default/trait.Default.html
  76 [debug]: https://doc.rust-lang.org/std/fmt/trait.Debug.html
  77 [derive]: https://doc.rust-lang.org/reference/attributes.html#derive